Fuel cell systems are increasingly being used as a power source in a wide variety of applications. Fuel cell systems have been proposed for use in power consumers such as vehicles as a replacement for internal combustion engines, for example. Such a system is disclosed in commonly owned U.S. Pat. No. 7,459,227, hereby incorporated herein by reference in its entirety. Fuel cells may also be used as stationary electric power plants in buildings and residences, as portable power in video cameras, computers, and the like. Typically, the fuel cells generate electricity used to charge batteries or to provide power for an electric motor.
Fuel cells are electrochemical devices which combine a fuel such as hydrogen and an oxidant such as oxygen to produce electricity. The oxygen is typically supplied by an air stream. The hydrogen and oxygen combine to result in the formation of water. Other fuels can be used such as natural gas, methanol, gasoline, and coal-derived synthetic fuels, for example.
The basic process employed by a fuel cell is efficient, substantially pollution-free, quiet, free from moving parts (other than an air compressor, cooling fans, pumps and actuators), and may be constructed to leave only heat and water as by-products. The term “fuel cell” is typically used to refer to either a single cell or a plurality of cells depending upon the context in which it is used. The plurality of cells is typically bundled together and arranged to form a stack with the plurality of cells commonly arranged in electrical series. Since single fuel cells can be assembled into stacks of varying sizes, systems can be designed to produce a desired energy output level providing flexibility of design for different applications.
A common type of fuel cell is known as a proton exchange membrane (PEM) fuel cell. The PEM fuel cell includes three basic components: a cathode, an anode and an electrolyte membrane. The cathode and anode typically include a finely divided catalyst, such as platinum, supported on carbon particles and mixed with an ionomer. The electrolyte membrane is sandwiched between the cathode and the anode to form a membrane-electrode-assembly (MEA). The MEA is disposed between porous diffusion media (DM). The DM facilitates a delivery of gaseous reactants, typically the hydrogen and the oxygen from air, to an active region defined by the MEA for an electrochemical fuel cell reaction. Nonconductive gaskets electrically insulate the various components of the fuel cell.
When the MEA and the DM are laminated together as a unit, for example, with other components such as gaskets and the like, the assembly is typically referred to as a unitized electrode assembly (UEA). The UEA is disposed between fuel cell plates, which act as current collectors for the fuel cell. The UEA components disposed between the fuel cell plates are typically called “softgoods”. The typical fuel cell plate has a feed region that uniformly distributes the gaseous reactants to and between the fuel cells of the fuel cell stack. The feed region may have a broad span that facilitates a joining of the fuel cell plates, e.g., by welding, and a shifting of flows between different elevations within the jointed plates. The feed region includes supply ports that distribute the gaseous reactants from a supply manifold to the active region of the fuel cell via a flow field formed in the fuel cell plate. The feed region also includes exhaust ports that direct the residual gaseous reactants and products from the flow field to an exhaust manifold.
The stack, which can contain more than one hundred plates, is compressed, and the elements held together by bolts through corners of the stack and anchored to frames at the ends of the stack. In order to militate against undesirable leakage of fluids from between the plate assemblies, a seal is often used. The seal is disposed along a peripheral edge of the plate assemblies and selected areas of the flow paths formed in the plates.
When the sealing surfaces are uniformly flat and parallel, conventional seals may be employed between plates. One prior art solution involves separate three-dimensional engineered seals specifically shaped to conform to contoured surfaces. Such three-dimensional seals may be all metal, all elastomeric, or a combination thereof. However, these prior art seals may be prohibitively expensive. Additionally, these seals are sensitive to dimensional and environmental variation, which makes use thereof undesirable for full scale production. Engineered seals also require highly accurate placement during a production step. Since such engineered seals are not adhered to one of the plates, the seal may migrate prior to compression and anchoring in place, adversely affecting the sealability. To avoid migration, a metal shim or foil may be added to sandwich and sufficiently support the engineered seal against deflection. However, the use of metal shims is undesirable since the shims must have a strength and thickness that resists deflection of the seal under pressure. The shim must also be sufficiently bonded to the seal to inhibit separation therefrom over repeated fuel cell operation. Thus, the employment of metal shims undesirably adds to a complexity and cost of the fuel cell.
Newer elastomeric seal materials make it possible to directly dispense a flowable sealant onto one of the plates, generally through an automatically controlled nozzle. However, the geometry of the fuel cell plates requires that the fluids being sealed follow a tortuous flow path through the fuel cell. The tortuous flow path typically includes open areas which reduce a velocity of the flow of the fluids, as well as reduced area flow paths created by surface features of each plate, thereby introducing three dimensional surfaces to be sealed. Such surface features also introduce areas to be sealed having varying thicknesses, thereby requiring dispensing non-uniform thicknesses of sealing material. Additionally, control of dispensing nozzles moving in three dimensions is difficult and costly, and the process of depositing the seal solely via dispensing nozzles is time-consuming, and is limited by the flowability of the sealant material. At higher linear speeds, the sealant exhibits undesirable undulations and pulling, reducing the deposition thickness. Because the dispensing speed must be limited, and because of the complex three-dimensional surface features on the fuel cell plate perimeter, the dispensing process requires an unacceptably long time period to accomplish, during which the uncured sealant is unnecessarily exposed to contamination. Also, because the sealant has some amount of flowability when in an uncured state, a longer elapsed time during sealant application may result in the sealant undesirably moving or changing shape prior to cure, again adversely affecting the seal integrity.
Further, because the sealing beads follow complex paths about the plates along a sealing surface, it is not possible to dispense the sealant as a single, continuous bead. Instead, multiple, discontinuous beads of sealant must be arranged to minimize the effects of breaks, knit lines, intersections and/or overlaps of the beads. Breaks between sealant beads reduce the integrity of the seal, while knits, intersections and overlaps of the beads may result in a wasteful surplus of sealant applied at a given location that also may adversely affect either the seal itself or the performance of the fuel cell stack, or both.
Therefore, it is desirable to obtain a formed-in-place seal assembly, and a method for its application, for sealing between plates of a fuel cell system, wherein the seal assembly and its manufacture militates against a leakage of fluids from the fuel cell system, facilitates a maintenance of a desired velocity of the fluid flow in the fuel cell system, and further addresses each of the aforesaid difficulties.